The present invention is directed to a method of forming thin films of silicon nitrides and other nitrogen-containing compositions, such as oxynitrides, directly on a silicon surface, and is further concerned with the application of such films.
Thin films of silicon nitride have two significant applications in the field of integrated circuits. Since they exhibit a higher dielectric constant and hence a higher unit capacitance than silicon dioxide layers, their use as capacitor dielectrics in small dimension MOS circuits is preferred. The increased unit capacitance that they exhibit makes it possible to fabricate capacitors in smaller areas and hence contributes to a denser circuit, relative to a circuit of similar complexity that employs a silicon dioxide film. In addition, silicon nitride films have greater resistance to radiation, alkali ion and other impurity diffusion.
A second application of thin silicon nitride films relates to the electrical isolation of integrated circuits by means of silicon dioxide islands. Typically, these islands are formed by oxidizing selected regions of a silicon substrate. Silicon nitride films are used to protect areas of the substrate where the devices are to be fabricated. In current practice, a thin oxide layer is used between the substrate and a deposited silicon nitride film to prevent the generation of stress-induced faults during oxidation. The presence of this oxide layer permits oxidization to proceed in a lateral, or horizontal, direction as well as in the vertical direction. Oxide encroachment in this lateral direction reduces the area that is subsequently available for fabricating the integrated circuit, and it is therefore preferable to minimize such. Accordingly, attempts have been made to form the thin nitride layer directly on the silicon substrate.
In the past, several different techniques have been employed to form a silicon nitride layer directly on the surface of a silicon substrate. One such technique is the thermal nitridation of silicon, which is carried out using ammonia or nitrogen gases at temperatures in the neighborhood of 1000.degree. C. or greater. This approach is disadvantageous in that it requires treatment at high temperatures for extended periods of time and is limited in the types of materials which can be present on or in the substrate. It is particularly unsuitable for use in the fabrication of VLSI devices, since the susceptability of dopants to diffusion at high temperatures presents problems with the small geometries that are involved. In addition, the resultant films contain a significant amount of oxygen, which hampers their effectiveness in resisting oxidation.
A variation of this technique involves plasma-assisted thermal nitridation with the use of inductively coupled reactors. This technique is disclosed, for example, in U.S. Pat. Nos. 4,277,320 and 4,298,629. A coil disposed around the reaction chamber generates an electromagnetic field that inductively heats the wafer to be coated and excites tne gas within the chamber to create a plasma. Such reactors have proven to be difficult to construct on a production-scale level, and are therefore not in widespread use. Furthermore, they operate at relatively high temperatures (close to 1000.degree. C.) that are produced by the inductive field, and hence have high power requirements.
Another technique for forming thin nitride films uses high energy ion implantation. This technique is generally not desirable from a commercial standpoint, since its throughput is limited by the relatively small ion beam that is employed. In addition, high current implantation systems are complex and expensive.
Low energy ion bombardment is a third technique that has been used to prepare nitride-like films on silicon. Limitations associated with this technique include the fact that processing can only be carried out on a single wafer at a time because of the beam size generated by presently available low energy ion bombardment sources, and the contamination of the resulting layer with materials that are used to fabricate the ion gun source.
A fourth technique for forming silicon nitride films is low pressure chemical vapor deposition (LPCVD). Thickness control of thin films is difficult with this process, and hence the films tend to be relatively thick, in the neighborhood of 300 angstroms or more. As a result, they are not suitable for use in trench isolation when in direct contact with silicon because they can produce stress at the corners of the trench, which leads to defects in adjacent regions of the silicon substrate.